Bearing having an array of microasperities

ABSTRACT

A bearing having a generally flat planer surface of pearlite, and an array of microasperities of martensite on the planer surface, each microasperity including a gently sloping front surface in relation to the direction of movement of the bearing and a sharply sloping rear surface in relation to such direction of movement. A pulsed laser beam or other heat source is focussed on the bearing to produce the array with microasperities of controlled size and shape.

This is a division of Ser. No. 632,602, filed Nov. 17, 1975, now U.S.Pat. No. 4,047,984.

BACKGROUND OF THE INVENTION

This invention relates to corpuscular energy beam producedmicroasperities for lubrication. More particularly, this inventionrelates to an apparatus such as a laser apparatus or other producing abeam of corpuscular energy for producing controlled microasperities onsurfaces such as bearing surfaces.

In rotating machinery, and especially where rotating shafts operatepartly within a housing that contains fluid under pressure, dynamicseals are used. These dynamic seals consist of two basic parts, a rotorand a stator. The stator is fixed to the housing, whereas the rotorbears against the stator and is fixed to a shaft passing through thehousing. In high pressure and temperature applications, and especiallywhere leakage must be kept to a minimum the metal rotor rotates againsta wear resistant stator of carbon material. Of course, with the rotorheavily loaded against the stator to ensure good sealing, wear isenhanced. This results in frequent replacement of the seals, resultingin unwanted down time and cost.

Somewhat by accident it was found that thin films were forming betweenthe rotating surfaces of face seals in certain applications. These filmsapparently existed in spite of the high loading forces placed on theseals. It was also found that the presence of these thin films oflubricant on the seal during operation resulted in considerably longerseal life. This led to research into thin film lubrication mechanismsand the use of microasperities to produce such films of a controlledthickness.

Microasperities are small projections or protuberances intentionallyformed on one or both of the bearing surfaces of a seal. These smallprotrusions or bumps have been found to produce the desired thin film oflubricant and retain it during dynamic operation of the seal. With thislubrication system, cavitation of the lubricant film occurs at thetrailing edge of the microasperities and flow of lubricant around andover the microasperities produces a pressure distribution that supportsthe load by controlled oil film thickness, producing a separation ofrotor and stator. The separation produced by the controlled oil film inthe field of microasperities is much greater than with a smooth lappedsurface. This has the effect of reducing the localized and surfacegouging by small carbides, and therefore greatly reduces wear. Thevariables involved are the size and shape of microasperities, theviscosity of the lubricant liquid, the rotor velocity and the thicknessof the liquid film over the microasperities.

Microasperities have been formed in numerous ways, and predominantly bychemical milling of photoetched, previously finished surfaces.Additional techniques include lapping, coining, and other etchingtechniques.

These techniques have been somewhat refined to the point where there hasbeen a progression from randomly sized and shaped asperities to attemptsto produce homogeneous asperity surfaces. The homogeneous surfaces areof course more amenable to analytical prediction. Cylindrical asperitiesin a geometric array have been produced by using photoetchingtechniques. These asperities have a circular contact surface. Morerecently, triangular shapes have been considered. See, for example:Dennis Lee Otto, Triangular Asperities Control Seal Leakage andLubrication, Society of Automotive Engineers, Paper No. 740201, 1974.

However, these triangular asperities, as with the aforementionedcircular asperities, have flat plane contact surfaces which are thenecessary result of using chemical etching techniques wherein theunetched surface is merely masked by use of a coating. Other morecomplex shapes, such as pyramidal or ramp shaped, have been proposed,which would require new manufacturing techniques.

The present invention utilizes a means for producing a beam ofcorpuscular energy and a control means therefor, such as a beam chopper,for producing microasperities of a controlled size, shape and density onbearing surfaces. The beam of corpuscular energy is conveniently a laserbeam, an electron beam, or a spark discharge. As an alternative to amechanical beam chopper, the duration of the beam may be controlled byelectronically pulsing the beam. The shape of the microasperity producedis pyramidal, or ramp shaped. An abrupt frontal wall and a tapering rearwall of such shaped asperity produces a desired cavitation effect andconsequently enhanced seal dynamics.

OBJECTS OF THE INVENTION

It is therefore the primary object of this invention to producemicroasperities of a more uniform size, shape, and density.

It is a further object of this invention to produce asperities in acontrolled array of a ramp, or pyramidal, shape.

It is a still further object of this invention to producemicroasperities by using a controlled beam of corpuscular energy, suchas a laser beam.

It is yet another object of this invention to produce microasperities onbearing surfaces by using a controlled beam of corpuscular energy and acontrol means therefor.

Other objects of this invention will become more readily apparent from areview of the following description, having reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a laser system of the instant invention for producingmicroasperities on bearing surfaces;

FIG. 2 is a view taken along lines II--II in FIG. 1, and showing thedetails of the mechanical laser beam chopper;

FIG. 3 is a graph illustrating the effect of the beam chopper on laserintensity against time;

FIG. 4 is an enlarged view of a portion of a bearing surface showing anarray of conventional circular microasperities thereon;

FIG. 5 is an enlarged side elevation view of a single cylindricalmicroasperity having a circular contact surface;

FIG. 6 is a graph illustrating the pressure distribution across theasperity shown in FIG. 5;

FIG. 7 is a top plan view of a portion of a bearing showing an array ofthe asperities of the instant invention;

FIG. 8 is a side elevation view in cross section showing a singlepyramidal or ramp shaped asperity of the instant invention andillustrating the bearing structure thereof;

FIG. 9 is a graph illustrating the pressure distribution across thepyramidal asperity of FIG. 8;

FIG. 10 is a side elevation view in partial section of a portion of ashaft housing showing a rotor-stator seal arrangement; and,

FIG. 11 is a view taken along lines XI--XI in FIG. 10 showing theasperity array.

DETAILED DESCRIPTION

Turning to FIG. 1, there is shown a corpuscular energy beam system forproducing microasperities according to the instant method and system.The corpuscular energy beam system shown generally at 10 comprises abeam generator 12 such as an HPL-10 laser produced by the Avco EverettCorporation and rated at 10 KW. It should be understood that the presentinvention is broadly directed to use of a corpuscular energy beam,including but not limited to a laser beam, electron beam, or a sparkdischarge. While the following discussion talks in terms of a laser,such is merely for the sake of convenience. The laser produces acontinuous columnated beam or pulse of cylindrical nature having adiameter D_(o). Focusing mirrors 16 intercept the beam 14 and convergeit into the beam 18 having a minimum spot size of 0.013 cm diameter withthe beam divergence of plus or minus 64.2 mrad. at a point 20 on aworkpiece 22. Workpiece 22 may be a flat bearing surface such as a rotoror stator. A work table 24 is rotatably mounted on a vertically movablecarriage 26 by means of a shaft 28. A motor (not shown) at 30 incarriage 36 is used to drive shaft 28 and thereby work table 24 torotate the workpiece 20. The motor is shown generally at 32 andworkpiece support housing 34 serves to translate work table 24.

In order to obtain desirably shaped and sized microasperities, a meansfor shaping such asperities in the form of a beam chopper system showngenerally at 36 is provided. Beam chopper system 36 comprises a roundpolished copper disk 38 of radius R₁ mounted at an angle A equal toapproximately 45°. A motor 40 rotates the copper disk 38 through a shaft42 connected to the center thereof. The copper beam chopper serves tonormally direct beam 14 into a beam dump 44. As best seen in FIG. 2, thedisk 38 contains therein a crescent shaped hole 46. The crescent shapedhole 46 is sized such that the entire beam width D_(o) of FIG. 1 passestherethrough at its widest portion 48. Hole 46 narrows to a rearwardpoint 50 where the beam is entirely chopped.

The beam intensity at point 20 on the workpiece 22 as a function of timeis shown in FIG. 3. The geometry of the hole 46 shown in FIG. 2 producesa variable intensity laser pulse as shown by the curves in FIG. 3. Thebeam intensity raises from zero to a maximum value (I_(max)) and thendecays to zero again during the time from t₁ to t₂. After a period ofzero intensity, the beam again repeats a pulse cycle from t₃ to t₄. Thisproduces a variation in the depth of hardening of the surface on whichthe laser beam impinges, thereby producing a series of ramp orpyramidally-shaped asperities, as will be hereinafter described.

As an alternative to using a mechanical chopper system 36 and beam dump44 as above described, the corpuscular energy beam may be shapedelectrically. For example, a low powered laser with shaped pulsesproduced electrically would eliminate the need for a mechanical chopper.As a further alternative, a multiple image lens system 47 could be addedwhich would separate the single beam into a plurality of spaced beams sothat a plurality of asperities would be simultaneously produced. Thiswould considerably shorten the time required to cover a surface withasperities.

FIG. 4 shows a conventional asperity or array pattern produced bychemical etching. The pattern is comprised of a plurality ofhomogeneously-spaced cylindrical asperities 52 having flat, circularcontact portions 54. A profile of a single asperity 56 is shown at FIG.5, when the top surface portion 54 is spaced a distance h from anadjacent bearing surface 58 by means of an oil or lubricant film 60.

FIG. 6 is a graph of the lubricant or fluid pressure corresponding withpoints in front of, on, and behind or downstream of the asperity of FIG.5, using a base of ambient pressure. As shown in FIG. 5, the pressuredistribution extends beyond the front and back edges 61,62,respectively, of the asperity. The pressure profile ranges betweenmaximum (P_(max)) at the leading edge at a distance L₁ to a minimum(P_(min)) at the back edge at a distance L₃. Ambient pressure (P_(amb))is present at the center at a distance L₂. The arrow 64 shows thedirection of movement of the asperity with respect to the stationarybearing surface 58.

Turning to FIG. 7, there is shown a top view of an array of ramp orpyramidally-shaped asperities 152 in a homogeneously spaced array. Theasperities are generally triangularly shaped and are in staggered rowsand columns with interstices 110 therebetween. The asperities eachinclude a gradually tapered and gently sloping front portion or surface160 and a steep or sharply sloping back or rear portion or surface 156.Tapered side portions or surfaces 112,114 which gradually taper fromfront leading edge 160 define a generally pyramidally-shaped or rampshaped asperity. As best seen in FIG. 8, the asperities produced on thegenerally planar bearing surface 116 are caused at the point ofimpingement of the laser beam. The bearing surface may be metal such asunhardened carbon steel which expands upon hardening. By laser heatingand allowing the material to self-quench, an asperity such as that shownis produced. A hardened teardrop of martensite 117 is produced at thecenter in the surrounding bearing, which is pearlite 118. The transitionregion 120, which is a combination of pearlite and martensite, is formedbetween the martensite and pearlite regions. A hardened spot of 0.01inches would produce an asperity of 0.001 to 0.000001 inches dependingon the type of steel used.

In operation, the bearing surface 116 moves in the direction 164 withrespect to stationary surface 158. As best seen in FIG. 9, as themembers slide relative to one another a high pressure builds up over theface of the microasperity because the confined liquid is highlyincompressible and produces a pressure profile shown. Cavitation occursbehind or on the downstream side of the microasperity as shown at 122,when P_(min) equals the vapor pressure of the lubricant. The net effectis an overpressure; that is, a pressure above the ambient pressure thatlifts and supports the moving member with respect to the stationarymember.

Turning to FIG. 10, there is shown an application of the microasperitiesof the instant invention. In the application a portion of a shafthousing 210 is partially cut away to show a stepped shaft 212 extendingthrough a shaft bore 214 in the housing. Rotary seal 216 is comprised ofa flat ring-shaped rotor 218 which bears against a cup-shaped stator 220fitted within an accommodating groove 222 in the housing. The rotor andstator seal along a circular line of contact 224. O-ring seal 226 islocated in an accommodating groove in the housing or sealing stator.

As seen in FIG. 11, the rotor ring contains a plurality ofmicroasperities in an array of concentric circles 228. As seen in FIG.1, these concentric circles are produced by the method of rotating thering by means of motor 30 in a vertically movable workpiece support 26.By indexing workpiece support 26 slightly, different concentric rings ofasperities can be produced. The method involves impinging the focusedlaser beam on the bearing surface of a preselected varying intensitywhile moving the body as described. If, on the other hand, arectangularly or other shaped piece is to be covered, the rotation couldbe stopped and workpiece support 26 indexed horizontally as well asvertically.

It is to be understood that the foregoing description is merelyillustrative of a preferred embodiment of the invention, and that thescope of the invention is not to be limited thereto but is to bedetermined by the scope of the appended claims.

What is claimed is:
 1. A bearing, comprising:(a) a generally planarsurface; and (b) an array of microasperities on said planar surface,each of said microasperities having a gently sloping front surface, anda sharply sloping rear surface.
 2. A bearing according to claim 1wherein each of said microasperities further comprises a pair of taperedside surfaces extending between said front surface and said rearsurface.
 3. A bearing according to claim 1 wherein said planar surfaceis pearlite and each of said microasperities is martensite.
 4. A bearingaccording to claim 3 further comprising a transition region of acombination of pearlite and martensite between said pearlite and saidmartensite.
 5. A bearing according to claim 1 wherein saidmicroasperities are spaced apart from one another to provide intersticestherebetween.
 6. A bearing according to claim 1 wherein said arrayincludes staggered rows and columns of said microasperities.